Caffeine is an alkaloid. There are numerous compounds called alkaloids, among them we have the methylxanthines, with three distinguished compounds: caffeine, theophylline, and theobromine, found in cola nuts, coffee, tea, cacao beans, mate and other plants. These compounds have different biochemical effects, and are present in different ratios in the different plant sources. These compounds are very similar and differ only by the presence of methyl groups in two positions of the chemical structure. They are easily oxidized to uric acid and other methyluric acids which are also similar in chemical structure.

Caffeine increases the level of circulating fatty acids. This has been shown to increase the oxidation of these fuels, hence enhancing fat oxidation. Caffeine has been used for years by runners and endurance people to enhance fatty acid metabolism. It's particularly effective in those who are not habitual users.

Caffeine and Osteoporosis (Calcium loss):
From the Journal of AMA: (JAMA, 26 Jan. 1994, p. 280-3.):
"There was a significant association between (drinking more) caffeinated coffee and decreasing bone mineral density at both the hip and the spine, independent of age, obesity, years since menopause, and the use of tobacco, estrogen, alcohol, thiazides, and calcium supplements [in women]."
Except when:
"Bone density did not vary [...] in women who reported drinking at least one glass of milk per day during most of their adult lives."

From Desk Reference to the Diagnostic Criteria from DSM-3-R (American Psychiatric Association, 1987): Caffeine-Induced Organic Mental Disorder 305.90 Caffeine Intoxication (overdose):
Recent consumption of caffeine, usually in excess of 250 mg.and
At least five of the following signs:
1. restlessness
2. nervousness
3. excitement
4. insomnia
5. flushed face
6. diuresis
7. gastrointestinal disturbance
8. muscle twitching
9. rambling flow of thought and speech
10. tachycardia or cardiac arrhythmia
11. periods of inexhaustibility
12. psychomotor agitation
Not due to any physical or other mental disorder, such as an Anxiety Disorder.
Basically, overdosing on caffeine will probably be very very unpleasant but not kill or deliver permanent damage. However, People do die from it.
Symptoms of caffeine withdrawal:
Regular caffeine consumption reduces sensitivity to caffeine. When caffeine intake is reduced, the body becomes oversensitive to adenosine. In response to this over- sensitiveness, blood pressure drops dramatically, causing an excess of blood in the head (though not necessarily on the brain), leading to a headache.